The present invention relates to optical communications networks, and in particular to resilient optical communications networks.
Broadband services are commonly supplied over legacy telephony networks using digital subscriber line (DSL) technology. One of the limitations of DSL is that the maximum data rate is not that high (for example, the maximum bit-rate is around 20 Mb s−1) and that the available bit rate decreases as the distance between the customer and the telephone exchange increases. It is believed that future broadband services are increasingly likely to be offered over optical fibre infrastructure using passive optical network (PON) architectures (D. B. Payne and R. P. Davey, “The future of fibre access systems”, BTTJ, Vol. 20-4, pp 104-114, (2002)). There are a number of large scale PON deployments currently underway around the world using either IEEE EPON technology or FSAN/ITU BPON and GPON. There is an emerging trend from network operators to consolidate network nodes and to reduce the amount of real estate used, leading to reductions in operational costs. A consequence of centralising network equipment into a smaller number of network nodes is that the network is more vulnerable to large scale outages in the event that a node is rendered out of service due to some catastrophic fault (fire, earthquake etc.).
According to an aspect of the present invention there a communications network comprising first and second optical line terminals and one or more optical network terminals wherein: i) the first optical line terminal has a first network connection to the one or more optical network terminals and the second optical line terminal has a second network connection to the one or more optical network terminals; ii) a virtual LAN connection is made to each of the one or more optical network terminals via the first network connection; and ii) a virtual LAN connection is made to each of the one or more optical network terminals via the second network connection.
Preferably the network is connected to one or more communications servers, wherein the virtual LAN connections established in steps ii) & iii) are extended to the one or more communications servers. The first network connection may be an active connection used to transmit data to and/or receive data from the one or more optical network terminals whilst the second network connection may be a standby connection. In use, a decision may be made to switch from the active first network connection to the standby second network connection for one or more of the optical network terminals. This decision to switch from the active connection to the standby connection may be made in response to network performance indicators associated with the virtual LAN connection made via the first network connection. The virtual LAN connection may comprise an Ethernet VLAN and the network performance indicators may comprise one or more Ethernet continuity check messages. Furthermore, in use, if the network switches from the active connection to the standby connection for a non-responsive optical network terminal, continuity check messages may be sent to one or more selected optical network terminals which are connected to the same network infrastructure as the responsive optical network terminal.
There are protection mechanisms already defined in PON standards e.g. G.984 series, but these assume that the standby OLT resides at the same central office location, which is not acceptable when protecting against catastrophic failures. Hence new mechanisms need to be defined for this network architecture. Furthermore, higher network layers (Ethernet, IP . . . ) need to be able to switch traffic to the correct OLT in the event of a failure. The responsibility for the management and operation of these layers may reside with independent service providers, with no direct access to the PON management system. The present invention provides a network structure that enables dual homing protection with a standard GPON product and also a mechanism for the higher layers to react when there has been a traffic switch to a protection OLT.
The solution proposed here for the provision of resilient services over a dual-homed GPON infrastructure involves protection at two levels: the physical layer protection of the PON itself and protection switching of services at the Ethernet layer. The physical layer protection of the PON is handled by the GPON management system and the service protection switching relies on recently ratified Ethernet Operations, Administration and Maintenance (OAM) standards for Connectivity Fault Management (IEEE 802.1ag/ITU-T Y.1731).